Split screen doesn't work on a map that isn't built to support it, and none of the base maps have shipped with multiplayer support enabled to start with (except versus).

To enable mutli-player/splitscreen on a map that is currently single player:

1. Make sure you start the game with a game controller plugged in

2. Add a second player controlled character to the level
a. F1 -> Load -> Spawn in a character
b. Hit "C" to make sure that the newly spawned character doesn't attack the original player character
c. Hit "L" to reset the level to its starting position (or last checkpoint, for checkpoint enabled maps)
d. Select that character (double click) and hit CTRL + SHIFT + P
e. Make sure the main player character and the newly spawned character are on the same team (U -> team -> make sure they share at least one team in that list)

3. Enable split screen from the settings menu at the top. F1 -> Settings -> Game -> Split screen. The split screen setting on the pause menu isn't working right now

4. Go back to a player-controlled state instead of the editor state. Hit "8" while in editor mode